I hate to be a Debbie Downer but cops can guess and it sticks. Exact same situation that our rider is facing, cop clearly didn't pace the person and it still stuck. All the way to the Ohio Supreme Court. I would still try and fight it though, the cop lied on the ticket..he said 'pace' he didn't say 'estimate'.
http://www.cleveland.com/open/index.ssf/2010/06/police_officers_visual_estimat.html
Would love to cross examine that cop. 'Clearly you were lying when you wrote the ticket, or you are lying now. How is the court to know which is which regardless of your answer'.
That alone would be worth the price of court costs.